Field of the Invention
The present invention relates to a process for recovering organic acids, such as acetic acid or butyric acid, from aqueous source solutions such as fermentation broths.
At the present time there are two basic synthetic methods for the preparation of acetic acid for industrial use. The first comprises the oxidation of natural gas to methanol followed by reaction of methanol with carbon monoxide over a noble metal catalyst to form acetic acid. The second, and older method of production of acetic acid, comprises oxidizing ethylene oxide to acetic acid. Both of these processes are based on natural gas and the cost is expected to increase markedly in the next few years.
All consumable acetic acid is prepared by the vinegar process which comprises the two steps of (1) fermenting glucose in the presence of microorganisms to produce ethanol and (2) oxidizing ethanol to acetic acid. One mole of glucose produces two moles of acetic acid. At present this process is far too expensive for industrial acetic acid production.
More recently, organisms such as Clostridium thermoaceticum have been discovered that can convert carbohydrate to acetic acid in yields of 80% or greater (100% in theory). Many organisms, which are attractive from a fermentation yield basis, cannot tolerate highly acid conditions. Hence, the organic acids must be partly neutralized as they are formed. This creates the problem of isolating a pure acid from an aqueous solution where it occurs primarily as the acid salt. In other cases as well, such as chemical process streams or waste streams, it is necessary to recover pure acids from aqueous solutions where the acid occurs primarily as the salt.
The approach used in the past has been to add a strong mineral acid, such as sulfuric acid, to the organic salt solution, and extract the resulting organic acid into an organic solvent for eventual recovery by distillation. This "consumables" approach necessitates adding an alkali to the fermentation liquor (or other solution) to maintain neutrality during culture (or other processing), adding sulfuric acid to the "waste liquor" to permit organic acid extraction, and then disposing of the resulting waste salt. In the case of theoretical glucose fermentation to acetic acid, the overall materials are:
2 glucose.fwdarw.6 acetic acid+6NaOH.fwdarw.6NaAc+6H.sub.2 O, 3H.sub.2 SO.sub.4 +6NaAc.fwdarw.6HAc+3Na.sub.2 SO.sub.4. By weight: 360 g glucose+240 g NaOH+294 g H.sub.2 SO.sub.4 .fwdarw.360 g HAc+426 g Na.sub.2 SO.sub.4 +108 g H.sub.2 O.
The present process permits recovery of acids, such as acetic acid, from fermentation liquors without the use of any consumable salts or acids or the disposal problem for the resulting by-product salt. In the present process as applied to fermentation, all materials with the exception of the fermentation substrates are recoverable and recyclable.